Workers with the Pebble Mine project test drill in the Bristol Bay region of Alaska near the village of Iliamma in 2007. The U.S. Environmental Protection Agency is proposing restrictions that would essentially block development of a massive gold-and-copper prospect near the headwaters of a world-premier salmon fishery in Alaska.

JUNEAU, Alaska — The U.S. Environmental Protection Agency proposed restrictions Friday that would essentially block development of a planned massive gold-and-copper mine near the headwaters of a world premier salmon fishery in Alaska.

The announcement came as the EPA was being sued by Pebble Limited Partnership, the group behind the proposed Pebble Mine, and the state of Alaska for allegedly exceeding its authority.

The state and Pebble Partnership, which was created to design, permit and run the mine, argue the EPA should not be able to veto the project before a mine plan is finalized and evaluated through the permitting process. Pebble has asked that a judge block the EPA from taking any additional steps, but no ruling has been made.

The EPA said as part of its analysis it used plans filed by the mine’s owner, Northern Dynasty Minerals Ltd., with the U.S. Securities and Exchange Commission in 2011. That information indicated the Pebble deposit is likely to involve excavation of the largest open pit ever built in North America, reaching a depth that rivals that of the Grand Canyon at nearly a mile, the EPA said in its report.

The agency looked at three mine scenarios, one based on the worldwide median size deposit that contains copper-, gold- and molybdenum-bearing minerals, which was the smallest scenario analyzed, and two that it said were based on statements made by Northern Dynasty, of mine sizes of 2 billion tons and 6.5 billion tons.

The restrictions proposed by EPA are in line with the estimated impacts of the smallest scenario, which McLerran said is one-eighth the smallest size contemplated in Northern Dynasty’s filing. They include loss of at least 5 miles of streams with documented salmon or loss of 1,100 or more acres of wetlands, lakes and ponds that connect to salmon-bearing streams or tributaries.

The EPA said if the proposed restrictions were finalized, mining of the Pebble deposit would still be possible if the environmental impacts were smaller than those laid out. It’s possible those behind the Pebble Mine could come up with something different than what EPA analyzed.

The EPA called its analysis conservative, focused on the use of certain waters in the region for disposal of materials associated with mining the Pebble deposit. The agency said it did not include impacts associated with build-out and operation of a mine, like roads, pipelines and housing for workers, or potential effects of accidents or mine failures.

The EPA also said the proposal is specific to the Pebble deposit, to mining claims held as part of the Pebble project, and does not affect other deposits or claims.

In 2011, the EPA, petitioned by Alaska Native tribes and others to protect Bristol Bay, initiated a review that culminated in the finding earlier this year that large-scaling mining in the Bristol Bay watershed posed significant risks to salmon and Alaska Native cultures that rely on the fish. The agency later invoked a rarely-used process through which it could ultimately restrict or prohibit development of the proposed Pebble Mine to protect the fishery.